Fluid system having positive vertical hold means

ABSTRACT

A fluid system having a variable displacement fluid pump connected in a closed-loop fashion to a fluid cylinder having a piston and a pair of connecting rods extending from opposite sides of the piston and externally of the fluid cylinder with one rod supporting a vertically displacable load. A directional flow control valve connected to the inlet and outlet of the fluid pump selectively directs fluid to pressure chambers within the fluid cylinder on opposite sides of the piston to reciprocate the same within the fluid cylinder and displace the load. The lower pressure chamber of the fluid cylinder, which is exhausted during a downward stroke of the piston, is in communication with the pump via the directional flow control valve and a pilot operated check valve which, in its normally closed position, permits fluid flow from the pump outlet through the directional flow control valve to the lower pressure chamber while preventing fluid flow from the lower pressure chamber to the pump inlet and thereby maintaining the load at any desired vertical position. An independent source of fluid, maintained at a constant pressure, is selectively connected to the pilot operated check valve to open same to permit an unrestricted communication between the lower pressure chamber of the fluid cylinder and the directional flow control valve when the same is shifted to communicate high pressure fluid from the pump outlet to the upper pressure chamber of the fluid cylinder to move the load downwardly.

United States Patent 9 Kubik Apr. 30, 1974' FLUID SYSTEM HAVING POSITIVE[57] V l ERTICAL 2 MEA NS 'A fluid system having a variable displacementfluid 1 Inventor; 1 Kublkf 6809 Sprwe pump connected 'in a closed-loopfashion to a fluid Blfmmgham, Mlch- 48010 cylinder having a piston and apair of connecting rods [22] Filed: June 6, 1972 extending from oppositesides of the piston and exter- [63] Continuation-impart of Ser. No.91,949, Nov. 23,

1970, Pat. No. 3,713,291.

[52] US. Cl 60/444, 60/460, 60/461, 91/420 [51] Int. Cl. Flb /18 [58]Field of Search 60/446, 444, 460, 461, 60/406, DIG. 2; 91/420 [56]References Cited UNITED STATES PATENTS 1,955,154 4/1934 Temple 91/4202,653,626 9/1953 Finlayson 91/420 X 2,657,533 11/1953 Schanzlin et a]60/406 2,984,985 5/1961 MacMillin 60/471 3,126,706 3/1964 Dettinger60/460 3,214,91 l ll/1965 Kempson 60/444 3,366,016 l/l968 Anderson...91/438 3,680,313 8/1972 Bl'undage 60/460 Appl. No.: 260,283

Related US. Application Data Attorney, Weintraub Agent,

or Firm-Weiner, Basile and nally of the fluid cylinder with one rodsupporting a vertically displacable load. A directional flow controlvalve connected to the inlet and outlet of the fluid pump selectivelydirects fluid to pressure chambers within the fluid cylinder .onopposite sides of the piston to reciprocate the same within the fluidcylinder and displace the load. The lower pressure chamber of the fluidcylinder, which is exhausted during a downward stroke of the piston, isin communication with the pump via the directional flow control valveand a pilot operated check valve which, in its normally closed position,permits fluid flow from the pump outlet through the directional flowcontrol valve to the lower pressure chamberwhile preventing fluid flowfrom the lower pressure chamber to the pump inlet and therebymaintaining the load at any desired vertical position. An independentsource of fluid, maintained at a constant pressure, is selectivelyconnected to the pilot operated check valve to open same to permit anunrestricted communication between the lower pressure chamber of thefluid cylinder and the directional flow control valve when the-same isshifted to 5 Claims, 3 Drawing Figures /Z4 /ZZ l SOLE/Vd/fl W84 32SOLE/V010 1 50 /49 '29 2s 78 38/ 0 26 42 24 76 /4 6 74 pe/ME MOVIE 1122:t 2 a2 PJIMULE LE 'T /08 Q6 90 I 86 f5 fi 3 9 n ax/ukk I u g 92 swan/oFLUID SYSTEM HAVING POSITIVE VERTICAL HOLD MEANS CROSS-REFERENCE TORELATED APPLICATIONS The present patent application is related insubstance to US. Pat. No. 3,653,208 and is a continuation-in-part ofcopending US. Pat. application Ser. No. 91,949,

now US. Pat. No. 3,713,291 both of which are incor-- porated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to fluidsystems for controlling the movement of afluid cylinder and, in particular, the present invention relates to apositive hold system for selectively preventing movement of a fluidcylinder piston while the same is supporting a vertical load.

2. Description of the Prior Art Heretofore, open-loop circuits have beenconstructed with a positive piston hold for a fluid cylinder of the typenormally disposed in a vertical position and supporting a displacableload thereabove. Such openloop circuits have included a pilot operatedcheck valve that functions to prevent fluid from being ex'- hausted fromthe lower pressure chamber of the fluid cylinder. However, theadaptation of the teachings of the open-loop circuits to a closed-loopcircuit has resulted in a highly inefficient andnoisy'operation. In suchpreviously used closed-loop circuits, the pilot'operated check valve isadapted to become operative, that is, to move from a closed to an openposition to permit the exhaust of fluid from the lower pressure chamberof the fluid cylinder by communicating the operating pressure of thesystem to the pilot valve, all of which has necessitated restrictivevalving and flow control means in order to maintain a proper operatingsystem pressure that results in very high pressure losses andunnecessary heat dissipation. Prior attempts which have been-employed toovercome the aforementioned difficulties have resulted in elaborate andexpensive circuitry employing valving and restrictiveflow passages, allof which become unnecessary in light of the teachings of the presentinvention.

SUMMARY OF THE INVENTION The present invention, which will be describedsubsequently in greater detail, comprises a fluid system having a closedloop fluid system for selectively connecting the inlet and outlet of afluid pump to the ports of a fluid cylinder which is nor-mally disposedin a vertical position and supporting a vertical load. Control means,operable upon actuation, are adapted'to communicate the pilot operatedcheck valve to an independent source of fluid maintained at a constantpressure when the fluid pump outlet is connected to the fluid cylinderto lower the supported'load. It is therefore'an object of the presentinvention to provide a closed-loop fluid system forraising and loweringa vertical load and having means for-providing a positive hold ofthe-vertical load.

It is also an object of the present invention to provide a closed-loopfluid system which employs a pilot operated check valve to achieve apositive hold of a vertically supported load.

, .Other objects, advantages and applications of the present inventionwill become apparent to those skilled in the art of fluid systems whenthe accompanying description of several modes contemplated forpracticing the invention is read'in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring to the drawing and, in particular, to FIG. 1,there is illustrated a fluid system 10 constructed in accordance withthe principles of the present invention and comprising a variabledisplacement fluid pump 16 connected in a closed-loop manner by conduits18, 20, 22 and 24 to a main fluid cylinder 26; Incorporated in thesystem 10 is a conventional four-way, solenoid operated, springcentered, directional flow control valve 28 which is adapted to connectthe conduits 18 and 20 selectively to the conduits 22 and 24', or bepositioned tandem-center to allow communication between the conduits 1'8and 20 but prevent fluid communication between conduits 22 and 24. I

The fluid pump 16 may be any suitable, variable displacement fluid pumpsuch as the pump disclosed in the aforementioned United States patentsand, as explained in greater detail therein, thearnount of fluiddisplaced by the fluid pump l 6 is controlled by an internaldisplacement control mechanism which may be varied between minimumandmaximum flow positions by an external lever 14*operated by a controlfluid cylinder 12 which will be described in greater detail hereinafter.In the schematic example of FIG. 1, the pump 16 is considered forpurposes of explanation to be at a minimum displacement or minimum' flowposition when the lever 14 is rotated clockwise and is considered to bein a maximum displacement or maximum flow position when the lever 14 isrotated counterclockwise by the action of the control cylinder 12. Theshifting'of the lever 14 between the minimum and maximum flow positions,or any intermediate flow positions, and the rate at which the lever 14is so shifted,

controls the amount of fluid and the rate at which the fluid 'isdisplaced by the fluid pump 16 and thus controls the fluid cylinder 26,all of which is described in greater detail in the aforementionedLetters Patents, and since the same does not form the subject matter ofthe present invention, a further detailed description thereof is notnecessary. I

An example of one means for controlling the displacement of the variabledisplacement fluid pump 16 comprises a directional flow control valve 21adapted to selectively communicate a controltluid maintained at aconstant pressure from a supply pump 25 via a conduit 34 to either of apair of feed control valve 27 or 29 which, in turn, are respectivelyconnected to the ports of the fluid cylinder 12 by any suitable conduitsor the like. The fluid cylinder 12 is conventional in its constructionand comprises a fluid piston 41 having a connecting rod 30 that extendstherefrom for'coupling with the pump lever 14. A conventional pressurerelief valve 32 communicates with the supply pump conduit 34 andfunctions in the conventional manner to limit the pressure of the fluidbeing delivered from the supply pump 25. A prime mover, such as anelectric motor, schematically illustrated at 35, is mechanicallyconnected through a suitable coupling to the drive shaft of the fluidpump 16, which, in turn, drives the supply pump 25. The fluid pump 16communicates with a fluid reservoir 36 through the charge pump 25,valving 37 and 38 on the pump inlet, and a drain conduit 39, all ofwhich is described hereinafter.

Feed control valves 27 and 29 may be conventional in their constructionand have restricted passages 40 and 42, respectively, which areadjustable such that the feed control valves may be pre-setto supply anydesired flow rate over any desired range. The feed control valves 27 and29 further comprise, respectively, check valves 44 and 46 which permitfluid to by-pass the restricted passages 40 and 42, respectively, in onedirection of flow. Thus, when the control valve 21 is in the positionindicated, fluid flow is directed from the supply pump 25 through thefeed valve 29 via the check valve 46 and is communicated to one side ofthe fluid cylinder 12 to force the piston 41 to shift the sameleftwardlyto displace the pump lever 14 toward a minimum flow position.At the same time, the fluid at the opposite side of the piston isexhausted from the fluid cylinder 12 through the restricted passageway40 in the feed valve 27 and returned to the. reservoir 36 via controlvalve 21. When the direction of the control valve 21 is reversed todirect fluid through the check valve 44 of the feed control valve 27,fluid is communicated to thepressure chamber on the opposite side of thefluid cylinder piston 41 whereupon the fluid pressure exerts a forceagainst the piston 41 to shift the same rightwardly as viewed in FIG. 1to increase, the displacement of the fluid pump 16, while at the sametime, fluid on the opposite side of the piston 41 is exhausted throughthe restricted passage 42 of the .feed valve 29 and back to thereservoir 36 via the directional control valve 21. k

The supply pump 25 is designed to deliver fluid at a constant pressureto the feed control valves 27 and 29 and, as will be explained ingreater detail, the output fluid from the supply pump 25 is communicatedto a second solenoid operated directional control valve 50 via a conduit52 that is connected to the conduit 34. I

The main fluid cylinder 26 has a vertically disposed cylindrical housing74 having an internal bore 76 in which a cylindrically shaped piston 78is reciprocally mounted, dividing the internal bore 76 into two pressurechambers 80 and 82,"respectively, opposite the upper and lower sides ofthe piston 78. The opposite sides of the piston 78 have cylinder rods 84and 86 which extend through the opposite end walls and externally of themain fluid cylinder .26. The pressure chambers 80 and 82 of the fluidcylinder 26 respectively have fluid ports 88 and 90 which, in turn, arerespectively connected to the fluid conduits 24 and'22; The connectingrods 84 and 86 have equaldiameters and thus the effective pressureresponsive areas on the opposite sides of the piston 78 are equal. Thefluid cylinder 26 operates in a well known manner to support a verticalload 91 and when the piston 78 is moved in opposite directions withinthe cylinder bore 76 by the action of fluid pressure in the pressurechambers 80 and 82, the load 91 may be raised, lowered or maintained atsome selected position.

The supply pump 25, which may be any suitable pump, such as a gear pump,is in communication with the reservoir 36 through a supply conduit 92and a filter 96 for supplying'a replenishing fluid to the conduits 18and 20, respectively, by means of the spring biased check valves 37 and38, both of which function in a well known manner to replenish thesystem.

Upstream from the directional control valve 28, the conduits 18 and 20are respectively connected to the inlets of high pressure relief valves108 and 110 which, at a predetermined pressure, e.g., 3,000 PSI, willexhaust the fluid pressure from one of the conduits to the other conduitto prevent damage to' the system in the event of over-pressurization. I

As'aforeme'ntioned, in certain applications using a fluid cylinder tosupport the load 91, a positive hold feature is employed to prevent theload from moving downwardly, that is, from moving the piston 78downwardly toward the port 90. In the present invention, the

positive hold 'feature is accomplished by having a pilot operated checkvalve 120 which will be described'in greater detail hereinafter. Thecheck valve 120 is disposed in the conduit 22 having one port 132 (FIG.3) communicating with theport of the fluid cylinder 26 while the otherport 134 communicates with the directional flow control valve 28. Apilot conduit 122 communicates a pilot pressure chamber 121 (FIG. 3) ofthe check valve with the directional control valve 50 which, in'turn, isoperative when its solenoid 124 is ac,- tuated to communicate fluidpressure from the supply pump 25 to the pilot pressure chamber 121 toopen the check valve 120 Ma manner which will be described. When thesolenoid 124 of the valve SOis deactivated, the pilot conduit 122 andthusthe pilot pressure chamber 121 is communicated directly back to thereservoir 36 via a conduit 123.

Referring now to' FIG. 3, wherein the pilot operated valve 120 isillustrated as comprising. a housing having spaced parallel boresdefining ports 132 and 134. The port 132 communicates with the port 90of the fluid cylinder 26 while the port 134 communicates with thedirectional flow control valve 28 via the conduit 22. The ports 132 and134 are connected by a passageway 135 having a valve seat 136. A valvemember 138 is biased by spring 140 into a sealing engagement with thevalve seat 136, and thus during normal operation of the valve 120 whenthe solenoid 149 of the directional flow control valve 28 is activatedto direct fluid under pressure from the pump outlet'through conduit20"to the conduit 22-and the pilot operated valve port 134, the pressurewill exert a force against the valve member 138 to unseat the same andcommunicate with the fluid cylinder lower pressure chamber 82 whereinthe pressure exerts a force against the piston 78 to raise the load 91vertically, while at the same time, fluid from the. pressure chamber 80is exhausted through the conduit 24, the directional control valve 28,the conduit 18 and returned tothe inlet of the pump 16. When the othersolenoid 150 of the directionalcontrol valve 28 is energized such as tocommunicate pressure fluid from the pump outlet to the conduit 24' andthus to the upper pressure chamber 80, fluid pressure therein actsagainst the piston 78 urging the same downwardly so as to lower the load91. However, since the check valve 120 normally will not permit freefluid communication from the lower pressure chamber 82 back to the pumpinlet, via the direction control valve 28 and conduit 18, the load 91may not be lowered.

In order to permit the lower fluid chamber 82 of the main fluid cylinder28 to be exhausted back to the pump inlet, the pilot operated valve 120is provided with a piston member 151 slidably mounted in the pilotpressure chamber 121 which is separated from the port 134 by a partition152. The piston member is carried by a rod 153 slidably supported inbore 154 extending through the partition 152 and is adapted toabuttingly engage the valve member 138 to unseat the same when controlpressure is communicated to the pilot chamber 121 and acts against theface 155 of piston member 151. The piston member 151 is biased to theposition shown by a spring 156 while a chamber 158 behind the pistonmember 151 is connected to the reservoir 36 via a conduit 126. When thevalve member 138 is unseated, permitting communication between thecylinder chamber 82 and the port 134, the load 91- may be lowered. Sincethe control pressure delivered from the supply pump 25 is usually I00 to125 PSI and the system pressure may be as high as 4,000 to 5,000 PSI,the effective pressure responsive area A of the piston member 151 mustbe substantially greater than the effective pressure responsive area A"of the valve member 138. In the preferredembodiment the area A'is 40times the area A".

In operation, in order to raise the load 91, the sole noids 124 and 150,respectively, ofthe valves 50 and 28 are in an unactivated positionwhile the solenoid 149 of the directional control valve 28 is activatedto shift the directional control valve 28 to a-position directing fluidflow from the conduit into the conduit 22 through the check valve 120and into the lower pressure chamber 82 exerting a force against thepiston 78 raising the load 91 while the pressure in the upper pressurechamber 80 is exhausted through conduit 24, the directional controlvalve 28 and returned to the pump inlet via conduit 18. To lower theload 91, the solenoid 149 of the directional control valve 28 isdeactivated and solenoids 124 and 150, respectively, of the directionalcontrol valves 50 and 28 are activated, whereby fluid from the highpressure outlet port of the pump 16 flows via conduit 20, thedirectional control valve 28 and the conduit 24, into the upper pressurechamber 80 ofthe fluid cylinder exerting a force on the piston 78 urgingthe same downwardly while simultaneously therewith the fluid from thesupply pump at a constant pressure is directed via conduit 52, the di-'rectional control valve 50 and the conduit 122 to the In order to stopthe movement of the load 91, the sowhereupon the fluid pressure is notcommunicated to either chamber or 82 and at the same time fluid in thelower pressure chamber 82 is prevented from being exhausted therefrom,maintaining the load in the desired level.

The solenoid operated directional flow control valve 28 is well suitedfor low volume operations. However, when very high volume operation isbeing employed, a pilot operated directional control valve may beutilized in the system 10. Referring to FIG. 2, a pilot operateddirectional control valve is illustrated and is adapted to replace thedirectional control valve 28 and the directional control valve 50illustrated in FIG. 1 with all other components of the system remainingthe same and functioning in the same manner, and thus are notillustrated in FIG. 2. However, it is to be understood that a systemusing the directional control valve 160 illustrated in FIG. 2 employsall the other components illustrated in FIG. 1. The directional controlvalve 160, which is conventional in its construction, comprises a mainvalve section 162 having a spring centered spool (not shown) which isactuated to perform the desired fluid connections by hydraulic pistons164 and 166 disposed at the opposite ends thereof. Control pressure fromthe supply pump 25 is selectively directed to the pistons 164 and 165 onthe opposite sides of the valve section 162 by a solenoid operated pilotsection 168. In addition to communicating fluid pressure to the oppositesides of the valve section 162, the pilot pressure is communicated vialine 122 to the pilot pressure chamber 121 of the check valve 120 tooperate the same in the manner hereinbefore described. In operation, toraise the load 91 carried by the fluid cylinder 26, asolenoid of thepilot section 168 is deactivated while the solenoid 1720f the valve 164is energized to shift the valve 168 to communicate fluid pressure fromthe supply pump 25 to the piston 166 causing the valve section 162 to beshifted in such a manner as to direct fluid from the' high pressureconduit 20 through the pilot operated check valve 120 into the fluidlower pressure chamber 82 exerting a force against the piston 78 toraise the vertical load 91 while fluid in the pressure chamber 80 isexhausted through the conduit 24, valve section 162, to the inlet of thepump 16 via conduit 18. During this mode of operation, the piston 164and the pilotpressure chamber 121 are connected to the reservoir 36 viapilot valve section 168 and a conduit 173. In order to lower the load91, the solenoid 170 of the pilot section 168'is energized while thesolenoid 172 is deactivated, whereby control pressure from the supplypump 25 is communicated to the piston 164 shifting the valve section 162so as to communicate fluid pressure from the outlet of pump 16 to theconduit 24 and thus to the upper pressure chamber- 82 exerting a forceon the piston 78 to lower the load 91. Simultaneously, control pressurefrom the valve section 168 isdirected via conduit 122 to the the pilotsection 168 to close communication between the supply pressure and thepilot valve 120 and the valve section 162, thereby preventing theexhausting of fluid from the lower chamber 82 of the fluid cylinder 26.

It can thus be seen that the present invention has provided a fluidsystem which has a very simple and effective means for providing apositive hold to a fluid cylinder which is supporting a vertical load,the same being accomplished without the necessity of a complicatedcircuitry, not requiring the use of flow control valves and othersimilar restrictive means to establish pressure drops between the inletand outlet of the pump as has been 'the required need of circuitsheretofore employing a check valve on the closed-loop circuit.

It can also be seen that the present invention has provided a positivehold for a fluid system which is extremely simple in its operation andconstruction and thus inexpensive to manufacture and one which is highlyreliable and of long life. v

It can also be seen that the present invention permits the employment ofa closed-loop circuit that provides for the controlled raising andlowering of the vertically supported load at any desired rate ofacceleration or deceleration while at the same time providing a positivebraking of the load when desired.

Althoughvonly two' forms of the present invention have beendisclosed,-it is to be understood by those skilled in the art of fluidsystems that other forms may be adopted, all coming within the spirit ofthe invention and scope of the appended claims.

What is claimed is as follows:

1. A fluid system comprising;

a fluid pump having inlet and outlet means;

a fluid cylinder having reciprocating piston means and first and secondport means respectively communicating within said fluid cylinder on theopposite sides of said piston means;

, directional flow control valve means selectively operable to connectsaid pump inlet and outlet means to said first and second port means, ina closed loop fashion for reciprocating said piston means within saidcylinder;

pilot operated check valve means disposed between said directional flowcontrol valvemeans and one of said cylinder port means, said check valvemeans being normally operable to permit fluid flow from said directionalflow control valve means to said one port means and prevent'a reverseflow of fluid from said one port means to said directional flow controlvalve means;

an independent source of fluid pressure;

control valve means operable upon actuation for communicating saidindependent source of fluid pressure to said pilot operated check valveto open same and permit fluid flow from said one port means to saiddirectional flow control valve means; and' "means for actuating saidcontrol valve means when 2. The fluid system defined in claim 1, whereinsaid fluid pumpis a variable displacement pump; and further comprisingpressure responsive means for varying displacement of said pump; saidindependentsource of pressure being adapted to be selectivelycommunicated to said pressure responsive means for varying displacementof said variable displacement pump.

3. The fluid system defined in claim 2, wherein said directional flowcontrol valve means comprises a pilot operated directional flow controlvalve having a first spool means movable when pressure is selectivelycommunicated to the opposite ends thereof, and a pilot valve having aspool means for selectively communicating said control pressure to theopposite sides of said first mentioned spool means, said pilot valvespool being adapted to communicate said control pressure to said pilotoperated check valve when in a selected position.

4. The fluid system defined in claim 1', wherein said pilot operatedcheck valve comprises a passageway having a valve seat and a valvemember engagable thereto, said valve member being movable in onedirection to permit flow thereacross, said valve member having rod meansextending therefrom and engaging a piston member, said piston memberbeing exposable to said control pressure and adapted to cause said valvemember to unseat from said valve seat or permit fluid communication inan opposite direction through said check valve, the effective pressureresponsive area of said piston member of said pilot operated valvetending to open said pilot operated valve being 40 times greater thanthe effective pressure responsive area of said valve member tending toclose said pilot operated valve.

5. A fluid system comprising:

a fluid pump having inlet and outlet means;

a fluid cylinder having reciprocating piston means and first and secondport means respectively communicating within said fluid cylinder on theopposite sides of said piston means;

means selectively operable to connect said pump inlet and outlet meansto said first and second port means, in a closed loop fashion forreciprocating said piston means within said cylinder;

pilot operated check valve means disposed between said pump and one ofsaid cylinder port means, said check valve means being normally operableto permit fluid flow from said pump to said one port means and prevent areverse flow of fluid from said one port means to said pump;

an independent source of fluid pressure;

control valve means operable upon actuation for communicating saidindependent source of fluid pressure to said pilot operated check valveto open same and permit fluid flow from said one port means to saidpump; and

means for actuating said control valve means when said pump outlet iscommunicated to said other cylinder port means.

- II i =8

1. A fluid system comprising: a fluid pump having inlet and outlet means; a fluid cylinder having reciprocating piston means and first and second port means respectively communicating within said fluid cylinder on the opposite sides of said piston means; directional flow control valve means selectively operable to connect said pump inlet and outlet means to said first and second port means, in a closed loop fashion for reciprocating said piston means within said cylinder; pilot operated check valve means disposed between said directional flow control valve means and one of said cylinder port means, said check valve means being normally operable to permit fluid flow from said directional flow control valve means to said one port means and prevent a reverse flow of fluid from said one port means to said directional flow control valve means; an independent source of fluid pressure; control valve means operable upon actuation for communicating said independent source of fluid pressure to said pilot operated check valve to open same and permit fluid flow from said one port means to said directional flow control valve means; and means for actuating said control valve means when said directional control valve means communicates said pump outlet to said other cylinder port means.
 2. The fluid system defined in claim 1, wherein said fluid pump is a variable displacement pump; and further comprising pressure responsive means for varying displacement of said pump; said independent source of pressure being adapted to be selectively communicated to said pressure responsive means for varying displacement of said variable displacement pump.
 3. The fluid system defined in claim 2, wherein said directional flow control valve means comprises a pilot operated directional flow control valve having a first spool means movable when pressure is selectively communicated to the opposite ends thereof, and a pilot valve having a spool means for selectively communicating said control pressure to the opposite sides of said first mentioned spool means, said pilot valve spool being adapted to communicate said control pressure to said pilot operated check valve when in a selected position.
 4. The fluid system defined in claim 1, wherein said pilot operated check valve comprises a passageway having a valve seat and a valve member engagable thereto, said valve member being movable in one direction to permit flow thereacross, said valve member having rod means extending therefrom and engaging a piston member, said piston member being exposable to said control pressure and adapted to cause said valve member to unseat from said valve seat or permit fluid communication in an opposite direction through said check valve, the effective pressure responsive area of said piston member of said pilot operated valve tending to open said pilot operated valve being 40 times greater than the effective pressure responsive area of said valve member tending to close said pilot operated valve.
 5. A fluid system comprising: a fluid pump having inlet and outlet means; a fluid cylinder having reciprocating piston means and first and second port means respectively communicating within said fluid cylinder on the opposite sides of said piston means; means selectively operable to connect said pump inlet and outlet means to said first and second port means, in a closed loop fashion for reciprocating said piston means within said cylinder; pilot operated check valve means disposed between said pump and one of said cylinder port means, said check valve means being normally operable to permit fluid flow from said pump to said one port means and prevent a reverse flow of fluid from said one port means to said pump; an independent source of fluid pressure; control valve means opeRable upon actuation for communicating said independent source of fluid pressure to said pilot operated check valve to open same and permit fluid flow from said one port means to said pump; and means for actuating said control valve means when said pump outlet is communicated to said other cylinder port means. 